7 research outputs found
Statistical Network Analysis for Functional MRI: Summary Networks and Group Comparisons
Comparing weighted networks in neuroscience is hard, because the topological
properties of a given network are necessarily dependent on the number of edges
of that network. This problem arises in the analysis of both weighted and
unweighted networks. The term density is often used in this context, in order
to refer to the mean edge weight of a weighted network, or to the number of
edges in an unweighted one. Comparing families of networks is therefore
statistically difficult because differences in topology are necessarily
associated with differences in density. In this review paper, we consider this
problem from two different perspectives, which include (i) the construction of
summary networks, such as how to compute and visualize the mean network from a
sample of network-valued data points; and (ii) how to test for topological
differences, when two families of networks also exhibit significant differences
in density. In the first instance, we show that the issue of summarizing a
family of networks can be conducted by adopting a mass-univariate approach,
which produces a statistical parametric network (SPN). In the second part of
this review, we then highlight the inherent problems associated with the
comparison of topological functions of families of networks that differ in
density. In particular, we show that a wide range of topological summaries,
such as global efficiency and network modularity are highly sensitive to
differences in density. Moreover, these problems are not restricted to
unweighted metrics, as we demonstrate that the same issues remain present when
considering the weighted versions of these metrics. We conclude by encouraging
caution, when reporting such statistical comparisons, and by emphasizing the
importance of constructing summary networks.Comment: 16 pages, 5 figure
Group Analysis of Self-organizing Maps based on Functional MRI using Restricted Frechet Means
Studies of functional MRI data are increasingly concerned with the estimation
of differences in spatio-temporal networks across groups of subjects or
experimental conditions. Unsupervised clustering and independent component
analysis (ICA) have been used to identify such spatio-temporal networks. While
these approaches have been useful for estimating these networks at the
subject-level, comparisons over groups or experimental conditions require
further methodological development. In this paper, we tackle this problem by
showing how self-organizing maps (SOMs) can be compared within a Frechean
inferential framework. Here, we summarize the mean SOM in each group as a
Frechet mean with respect to a metric on the space of SOMs. We consider the use
of different metrics, and introduce two extensions of the classical sum of
minimum distance (SMD) between two SOMs, which take into account the
spatio-temporal pattern of the fMRI data. The validity of these methods is
illustrated on synthetic data. Through these simulations, we show that the
three metrics of interest behave as expected, in the sense that the ones
capturing temporal, spatial and spatio-temporal aspects of the SOMs are more
likely to reach significance under simulated scenarios characterized by
temporal, spatial and spatio-temporal differences, respectively. In addition, a
re-analysis of a classical experiment on visually-triggered emotions
demonstrates the usefulness of this methodology. In this study, the
multivariate functional patterns typical of the subjects exposed to pleasant
and unpleasant stimuli are found to be more similar than the ones of the
subjects exposed to emotionally neutral stimuli. Taken together, these results
indicate that our proposed methods can cast new light on existing data by
adopting a global analytical perspective on functional MRI paradigms.Comment: 23 pages, 5 figures, 4 tables. Submitted to Neuroimag
La réflexivité entre anecdote et raisonnement
Symposium conducted at the meeting of ARGAGE (Argumentation & Language)International audienc
La réflexivité entre anecdote et raisonnement
Symposium conducted at the meeting of ARGAGE (Argumentation & Language)International audienc